G.fast is much like VDSL in that it uses your existing copper twisted pair line from you home to the cab at the end of your street.
So you can either set up your DSL-AC88U to use G.fast via its DSL port (it has an inbuilt VDSL/ G.fast modem) or just use the DSL-AC88U WAN port and piggy back of the BT G.fast modem.
Alas I am not sure what kit Talk Talk will provide as in the early days a BT engineer visited and updated your BT faceplate to a BT G.fast Faceplate and then installed a BT or other G.fast modem
This was a while ago and ISP may have started to suplly there own modem/router combos etc.
If the BT engineer supplies you with a modem, then that is great and use it, but if you are given a modem / router combo unit, then I would try to use the modem inbuilt into the DSL-AC88U.
Either way, let us know what you end up with as I am sure many others are very interested in this little project 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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